Timer1c support and per-variant initialization function

These are two fairly unrelated improvements, other than that I need them both for a sleep timing improvement in our Pinoccio project.

The first commit adds support for timer 1c, on hardware that supports it. The second commit allows a variant file to supply a custom initialization function, to allow for board-specific code to be included by third parties.

@cmaglie, I'd be thankful if you could have a look at these soon and merge them if they look ok to you? I need these changes merged before I can publish some of my own Pinoccio work. Thanks!